1. Field of the Invention
This application relates to a shielding tape. More particularly, this application relates to a shielding tape for LAN (Local Area Network) cables.
2. Description of the Related Art
LAN or network type communication cables are typically constructed of a plurality of twisted pairs (two twisted conductors), enclosed within a jacket. A typical construction is to have four twisted pairs inside of a jacket, but many other larger pair count cables are available.
Care is taken to construct these cables in a manner to prevent cross talk with adjacent cables. For example, in a typical installation, many LAN cables may be arranged next to one another, and signals in the pairs from a first cable may cause interference or crosstalk with another pair in an adjacent LAN cable. In order to prevent this, the lay length or twist rate of the pairs in a cable are varied differently from one another. Additionally, when pairs in adjacent cables are running parallel to one another the cross talk can be increased so the pairs within a cable are twisted around one another (helically or SZ stranding) to further decrease interference. Spacing elements can also be used so that the jacket is spaced apart from the pairs so that pairs in adjacent cables are as far away as possible.
Nevertheless, despite all of these features, in some cases, the requirements for increased bandwidth may necessitate additional protection from crosstalk. One such common type of protection is shielding. LAN cable shielding is usually in the form of a foil that is wrapped around the pairs inside the cable, under the jacket. This metal foil is usually wrapped around the assembled core of twisted pairs prior to jacketing and is constructed of suitable metals, for example aluminum.
Although the shield is effective for preventing alien crosstalk and other external signal interferences, the shield must be grounded to the connector in order to meet safety regulations. This is a time consuming step that increases the cost to install the shielded cable. One typical example requires a drain wire to be helically coiled around the shield which also increases the overall cable cost.
In the prior art, there have been proposals to mitigate the above effect by providing a discontinuous shielding tape having periodic breaks in the shield. This design makes sure that any signals that collect in the shield do not extend continuously from end to end of the cable and this obviates the need for grounding the shield. However, in doing so, this design has generated yet another drawback, particularly with respect to the signal quality within the pairs of the cable, owing to interference caused by signals generated by the discontinuous shield elements.
For example, with discontinuous shields, the signals traveling in the pairs can cause induced signals in discontinuous foil elements with the breaks in the shielding giving rise to reflected waves which can create issues with return loss. The patches can collectively interact with the transmitting electrical signals in a cumulative or resonant manner to produce a spike in return loss at a particular frequency of the transmitting signals.
In one example, where the foil size and shape is rectangular with each foil element of the same size and at regular spacing from one another, the generated reflected waves are such that they may occur at one specific frequency, and at a significant amplitude.
Other prior art arrangements of discontinuous shields have attempted to minimize the reflected wave that can be created by discontinuous shielding elements of equal length and spacing by varying the length of the shielding elements relative to the length of the foil segments, finding that the frequency/location of the spike may depend upon the sizes of the foil sections and the gap therebetween.
Other prior art discontinuous shielding tapes try to minimize the amplitude of the reflected wave by having foil pieces (and breaks) that are not perpendicular to the long edge of the substrate running in the direction of the pairs (ie parallelograms).
Although these various arrangements may have some mitigating effect to reduce the amplitude of the reflected waves by increasing the range of frequencies that these reflections occur at, they are still not an optimum solution.